Conventional silver halide color photography depends upon the formation of dyes by the oxidative coupling of coupler compounds with oxidized developer compound that is generated by the reduction of silver halide during development. Often, to improve coupler efficiency, the coupler compound is provided with a coupling-off group to enable the coupler to form a dye with the concomitant theoretical consumption of only two rather than four moles of silver halide. Unless of the "wash-out" variety, the coupling-off group can remain in the photographic element subsequent to processing and should have little or no deleterious effect on the resulting image.
It has been known to employ a releasable dye or dye precursor as the coupling-off group to provide a so-called "high dye-yield" coupler and to thus improve the theoretical efficiency of the coupler. In such a manner, the coupler can provide two dye molecules instead of one. Further efficiency improvements in silver usage can thus be envisaged.
Useful high dye-yield (HDY) couplers have been disclosed by Mooberry and Singer in U.S. Pat. No. 4,840,884. Such couplers react with oxidized color developer to form one dye and in doing so release a second dye or a precursor of a second dye. In accordance with the patent, the new couplers described therein enable lower concentrations of silver halide in the photographic element without lowering image quality. It has been found, however, that the high dye-yield couplers taught in the Mooberry patent exhibit a number of disadvantages. The azo dye releasing couplers of the patent provide improvements in dye-forming efficiency over that obtained from conventional couplers but not to the extent necessary to justify the increased manufacturing costs associated with the manufacture of such couplers. Moreover, the characteristics of the dye formed by release are less than desired. Yellow azo dyes provide extinctions in the neighborhood of 30,000 compared to 20,000 for the azamethine dyes formed by the conventional yellow coupler. However, these extinctions are less than desired and the azo dyes generally exhibit broader absorption bandwidths which result in inferior hue.
Methine dyes provide corresponding extinctions in the neighborhood of 50,000 and can therefore provide more density if hue and stability problems can be overcome. Examples 3 through 6 of the Mooberry patent suggest methine dye chromophores for the released second dye but the features obtainable with the particular dyes suggested are less than desired. The exemplified couplers do not provide the optimum features of coupler stability and satisfactory dye hue.
It is a problem to be solved to provide a photographic element that incorporates a high dye-yield coupler that exhibits the desired features of coupler stability and satisfactory dye hue.